Purification of molten electrolytes



rates PURIFICATION 6F MQLTEN ELECTROLYTES William W. Gullett, College Park, Md., assignor to Chicago Development Corporation, Riverdale, Md'., a corpartition of. Delaware No Drawing. Application August 26, 1957 Serial No. 680,391

1 Claim; (Cl. 204-454) This invention relates to cell bathsfor. the refining of titanium. It relates in particular to the purification of such baths as are disclosed in-mycopending application, Serial No. 573,356, filed March 23, 1956, now Patent No. 2,817,631, and to baths having been treated for valence adjustment in accordance with my copending application, Serial No. 660,418, filed May 20, 1957.

A typical bath is composed of sodium chloride having dissolved therein 18% Ti as lower chlorides, average valence 205-27, sodium .1-5.0%. When such baths are replenished with lower titanium chlorides or when impure anodes are used there is a tendency for impurities more noble than titanium to accumulate in the bath. I have found that these are not always eliminated by the procedure for valence adjustment set forth in my copending application but require an electrolytic procedure using a pure titanium anode and a small cathode so that an apparent current density on the cathode substantially greater than the operating current density can be employed.

I electrolyze under these conditions for a short time only as otherwise the bath would be depleted in titanium. The cathode and its deposit are then separated from the bath and the impure finely divided cathode deposit recovered.

I employ such a clean-up electrolysis whenever the build-up of impurities causes the formation of fine crystals in the cathode deposit. I have not been able to determine all of the impurities which have the effect of producing such fine crystals; they include aluminum, iron and chromium; magnesium may also be troublesome. All of these and others unknown to me may be removed by the application of my present invention.

Having now described my invention in general terms, I will illustrate it by examples.

Example I In this experiment, I add to a molten bath (850 C.) of NaCl under an inert atmosphere, the product of the reduction of TiCL, with sodium as described in my copending application, Serial No. 660,418, filed May 20, 1957, so that I have an electrolyte composed of 5.2% soluble titanium having an average valence of 2.81. This bath also contained the impurities of l e-6.2%, Crl.8%. I pass a direct current of 20 amperes (10 amps/sq. ft. on the cathode area) for 20-h'ours until the open circuit voltage reads 20 millivolts, as described in my copending application. T his bath now analyzes 5.5% soluble titanium as lower chlorides having an average valence of 2.51 with a sodium metal content of 1.3%. The impurities in the bath now analyze Fe6.0%, Cr1.73%.

To this bath, 1 now suspend a pure 2" dia. titanium rod anode and a A" dia. iron rod cathode and pass a direct current so as to have an apparent current density of 2250 amps/sq. ft. on the cathode for a period of 20 minutes.

This bath now has an analysis of 5.1% soluble titanium as lower chlorides having an average valence of 2.49 and a sodium metal content of 1.01%. Further analysis atent ice 2 shows the Fe.043% and Cr.09%'. The'cathode product consisted of fine crystals containing 13.6% Fe, 3.1%

Cr, balance substantially titanium.

I then replace the anode and cathode with an anodeafter arc melting into a button has a Brinell hardness.

Example [I In this experiment, I use the electrolyte under an inert,

atmosphere as composed at theend of Example I and place in the. concentric. anode basket. sheet clippings of a commercial titanium alloy containing 6% Al and 4% vanadium. A /4" dia. iron rod used as the cathode is placed in the center of the concentric anode basket so that all surfaces of the cathode are equidistant from the surface of the anode material. I pass a direct current of 500 amps/sq. ft. through the cell for a period of 12 hours and remove the cathode product. After repeating this electrolysis four times and removing the cathode product each time, I find that the cathode product is a very fine dark powder having a Brinell hardness of 320. The salt now has an analysis of 4.9% soluble titanium as lower chlorides having an average valence of 2.47, a sodium metal content of 1.03% and the impurities of Fe---.038%, Cr-.08%, Al-l.9% and V.83%.

I then remove the anode and cathode used in these runs and replace with a 2" dia. pure titanium anode and a /2" dia. iron cathode. I then pass a direct current through the cell corresponding to 2500 amp./ sq. ft. apparent cathode current density for a period of one hour. I then remove the cathode and recover the cathode product. This material is powdery and has a B. H. N. of 335. The salt analyzes Fe-.02l%, Cr-.06%, Alnil, V-.075%, soluble Ti as lower chlorides4.73% having an average valence of 2.49 and a sodium metal content of 96%.

I then replace the 6 Al-4 V alloy in the concentric steel basket as an anode and a new /4" dia. iron cathode and pass a direct current through the cell to correspond to an apparent cathode current density of 500 amps/sq. ft. The cathode product recovered after 16 hours consisted of coarse crystalline intergrowths of titanium and has a B. H. N. of 65.

Example III in this example, I add to a molten bath of the eutectic mixture of NaCl and SrCl under an inert atmosphere, the product of the reduction of TiCl with sodium metal as described in my copending application, Serial No. 660,418, filed May 20, 1957, so that I have an electrolyte composed of 7.8% soluble titanium as lower chlorides having an average valence of 2.93. This bath also contains the impurities of P e-10.3%, Cr2.3%.

I use a cell as described in my copending application, Serial No. 660,418, filed May 20, 1957, and electrolyze for 24 hours until the open circuit voltage reached 22 millivolts. The bath then has an analysis of Fe9.3%. Cr2.1%, soluble titanium as lower chlorides-8.1%, an average Valence of 2.51 and a soluble strontium metal content of 4.9%. I then replace the anode with a circular concentric basket of commercially pure titanium sponge having a Brinell hardness of and in the center I place a /i" dia. cathode of iron. I then pass a current through the cell corresponding to 4000 amp./ sq. ft. for 10 minutes. Upon removal, the cathode product which is composed of fine crystals of titanium has a B. H. N. of

275 and the salt has an analysis of Fe-.039%,

Cr-.019%. The electrolyte" now had a soluble titanium content of 8.05% as lower chlorides having an average valence of 2.5lwith a soluble strontium metal content of Using the same titanium in a concentric basket with a fresh dia. iron rod, I pass a-current through the cell corresponding to 100 amps/sq. ft. for 14 hours. The

titanium product recovered has a B. H. N. of 62. The

salt analyzes Fe --.038%', Cr.02()%, soluble titanium of 8.1% as lower chlorides having an average titanium valence of 2.50 with a soluble strontium metal content of 5.2%. a

What is claimed is:

Ina process ofelectrorefining' titanium in a singlepure titanium to be refined to an inert solid cathode, the

steps which consist in purifying the contaminated bath, without substantially changing the composition of the same as regards said essential constituents, by substituting a pure titanium anode for said impure anode, passing a direct current through the electrolyte from said pure titanium anode to a substantially smaller inert cathode at a current density on the cathode, at least 2000 amps/sq. it, several times as great as that employed in the electrorefining operation, for a period of time, at least 10 minutes, sufficient to remove substantially all of the contaminating metal from the bath as a fine powderous alloy deposit, and thereupon replacing the impure anode and the cathode and continuing the electrorefining operation.

References Cited in the file of this patent UNITED STATES PATENTS 

